Flow cytometry is the process of analyzing and sorting cells in a flowing stream. This is accomplished by intersecting the stream with an incident light, usually a laser, and detecting the resulting scattered light and fluorescence of the individual cells as a function of the particular physical characteristics or attached fluorescent dye, respectively. In addition, electronic volume sensing has also been incorporated in some flow cytometry instruments. With this array of detectors, sub-populations of cells can be analyzed and sorted in arbitrary terms by just detecting their qualitative differences. However, without size and fluorescent standards, no quantitative information on the individual cells can be gained other than the number of them counted and their proportion relative to the rest of the sample.
To determine quantitative differences between subpopulations of cells, and moreover, to give individual populations a quantitative relevance, standards are necessary with known amounts of fluorescence to which these cell samples can be compared. In FIG. 1, a microbead containing a fluorescent dye, fluorescein isothiocynate (FITC), is shown along with a cell labeled with the same dye. If a series of such microbeads containing varying amounts of the fluorescent dye is run on a flow cytometer, the resulting distributions will be obtained, as shown in FIG. 2, indicated by "Bead 1, Bead 2, and Bead 3". Now if a cell population stained with the same dye is also run on the flow cytometer under the same conditions, then the fluorescence intensity of the cells can be quantitatively compared to those of the calibrated microbeads.
Various fluorescent particles have been used in conjunction with flow cytometry including fixed cells, pollen, fluorescent microbeads, and stained nuclei. However, their use has been limited for the most part to instrument alignment and size calibration. Quantitation of fluorescence intensity of cell samples has been hampered by not having a highly uniform, stable particle which has the same excitation and emission spectra as the cells being measured. Those particles which contain the proper dyes, e.g., the fluorescein stained nuclei (marketed as Fluorotrol-GF by Ortho Diagnostic Systems, Inc.) are not stable over long periods of time and those which are considered stable, e.g., the microbeads, have not contained the same dyes which label the cells. Highly uniform fluorescent microbeads have been available from various sources for a number of years. However, none of these beads have been suitable as quantitative standards for flow cytometry instruments because (1) many of these fluorescent microbeads are smaller than the cells to be analyzed and (2) the fluorescent dyes which have been incorporated into the small microbeads are different from those attached to the cells. Attempts have been made to cross-calibrate microbeads containing one dye against solutions or cells containing a different dye, e.g., cumerin containing microbeads against fluorescein solutions. However, such a calibration is only good for the one excitation and emission filter system. Use of slightly different filter systems, which may occur with instruments from different manufacturers, can significantly alter the quantitative results. The invention recognizes that the key to having a useful fluorescent standard which can be used on any instrument or filter system is for the microbead to have the same excitation and emission spectra as the sample. A more subtle point recognized by the invention is that the environment of the dye molecules can have a large effect on the fluorescence spectra. This is demonstrated in FIG. 3, where the emission intensity of a cell labeled with fluorescein is compared to that of microbeads with fluorescein on the surface and fluorescein within the body of the hydrophobic microbead. The surface fluorescenated microbead has the fluorescein in contact with the aqueous medium, and has the same emission properties as a function of wavelength as does the fluorescein labeled cell suspended in an aqueous medium, whereas, the microbead with the fluorescein within the hydrophobic bead body has a very different response because both the excitation and emission spectra have shifted and broadened as a function of the dye being in a hydrophobic medium and not in contact with water. The related spectra as recognized by the invention are shown in FIG. 4. Recently, larger microbeads suitable for size calibration of biological cells have been synthesized in outer space (see NASA TM 78132 "Large-size Monodisperse Latexes as a Commercial Space Product", and U.S. Pat. No. 4,247,434), as well as, in the laboratory (see U.S. Pat. No. 4,336,173). However, none of these large microbeads were reported to contain a fluorescent dye. Moreover, synthesis as described in U.S. Pat. No. 4,336,173 was found to be hampered by agglomeration and high doublet formation. Even with the suggested polymeric stabilizers, the yields of mono-dispersed microbeads were lower than acceptable. The present invention has for its object the accomplishment of this task. Other objects will appear as the description proceeds.